1. Field of the Invention
The present invention relates to a highly integrated magnetic transducer for recording data on a magnetic carrier. It is applicable particularly to high-density recording, in terms of linear or radial length, of data on magnetic discs in disc memories, and more particularly to recording on magnetic discs of magneto-optical memories used in data processing systems.
2. Description of the Prior Art
In magnetic disc memories, it is known for the data stored on the magnetic discs to be contained on the interior of concentric, circular magnetic recording tracks generally covering the greater part of the two faces of the disc.
A sequence of magnetic data written on one track of a disc generally takes the form of a succession of small magnetic domains, called "elementary domains", distributed over the entire length of the track and having magnetic inductions that are of the same module and face in the same directions.
The term "linear data density" is used to mean the number of changes in direction of magnetization per unit of length, measured along the circumference of a track, and the term "radial density" refers to the number of tracks per unit of length, measured along the diameter of the disc.
For the sake of simplicity, the term "transducer" is used for the means that enable either the writing (or, equally accurately, the recording) of the data on the magnetic discs or the reading of the data therefrom, or finally that enable the realization of both of these two functions. One or more transducers are generally associated with one face of a given disc, and the disc passes before the transducer or transducers as it rotates.
One recent trend in the development of magnetic disc memories is toward research into magneto-optical memories, where the data are written onto the magnetic discs by means which are most often magnetic or thermo-magnetic. Reading is effected by an assembly of opto-electronic devices which make it possible to observe one face of a disc, at a given moment and at a given site, by means of a beam of polarized light and to furnish an electrical signal, the voltage or current of which is a function of the value of the data located at that site.
In these memories, the goal is to attain radial and linear densities equal or superior to 5,000 tracks per centimeter and 10,000 changes in the direction of magnetization per centimeter, respectively.
The method of writing on recording data is thus selected for magnetic discs of magneto-optical memories is known as "perpendicular recording". In this method, the magnetization in the elementary domains is perpendicular to the magnetic recording layer of the disc. It is found that this type of magnetization makes it possible to obtain greater linear and radial data densities and that the type of observation associated with it, that is, with a beam of light, is simpler than the type of observation in a recording mode where the magnetization is longitudinal, or in other words parallel to the magnetic recording layer and to the track. In perpendicular recording, the magnetic material comprising the recording layer is an anisotropic magnetic material; that is, a material having at least one direction of privileged magnetization, also known as the "direction of easy magnetization".
In magneto-optical memories, one method of writing the data makes use of magnetic transducers generally comprising a magnetic circuit coupled to a winding and including an air gap. The variation in induction at the interior of the air gap enables the writing of the data contained on the carrier associated with this transducer.
In order to attain very high linear and radial data densities, integrated magnetic transducers are preferably used, of the type described in U.S. Pat. No. 4,287,544 of Jean-Pierre Lazzari, issued on Sept. 1, 1981 and entitled "Magnetic Data Carrier for Perpendicular Recording"; the patent is assigned to the Compagnie Internationale pour l'Informatique CII Honeywell Bull.
The integrated magnetic transducer includes two pole pieces realized in thin magnetic layers and disposed on the same side of the data carrier, forming an air gap in the vicinity thereof. The pole pieces enclose an electrical winding formed of thin conducting layers superimposed on one another and separated from one another by thin insulating layers. The transducer rotates before the carrier perpendicular to the plane of the two thin magnetic layers comprising the pole pieces. Upon this rotation, every magnetic domain of one track of the carrier facing which the transducer is disposed passes in succession over time to face the first pole piece, called the "upstream pole piece", and the second pole piece, called the "downstream pole piece". Preferably, if the width of the pole pieces is considered to be the dimension thereof measured parallel to the direction of rotation, then the width of the upstream pole piece is substantially greater than that of the downstream pole piece (generally by more than a factor of 5).
Writing data on the carrier is accomplished by causing the carrier to rotate at a given constant speed and by causing a variable current which is representative of the data to be written to pass through the winding. This current, which passes through all the conducting layers of the winding, causes a magnetic flux to be generated in the pole pieces which closes across the magnetic layer of the data carrier. The magnetic flux is concentrated opposite the downstream pole piece because the width of this pole piece is so small. As the axis of easy magnetization of the magnetic layer is perpendicular to the surface of the magnetic recording layer, the component of the magnetic field perpendicular to this surface has an intensity sufficient to cause the reversal of the magnetization in this direction. Opposite the upstream pole piece, by contrast, the magnetic field disperses, and its component perpendicular to the surface of the layer has an intensity which is much less than the same component opposite the downstream pole piece. This makes it possible not to modify the magnetic state of the layer at the level of the upstream pole piece and enables the downstream pole piece to write the data under the most advantageous conditions.
The pole pieces are preferably realized in an anisotropic magnetic material, the axis of easy magnetization of which is perpendicular to the direction in which the data rotate and parallel to the surface of the carrier, and the axis of difficult magnetization is perpendicular to the data carrier. The advantages of using anisotropic pole pieces are set forth particularly clearly in U.S. Pat. No. 3,723,665, also of Jean-Pierre Lazzari, and assigned to the Compagnie Internationale pour l'Informatique and the Commissariat a l'Energie Atomique, issued on Mar. 27, 1973 and entitled "Integrated Magnetic Head Having Alternate Conducting and Insulating Layers Within an Open Loop of Two Magnetic Films".
With a view to attaining radial and linear densities on the order of those indicated above, it is necessary for the dimensions of the downstream pole piece to be such that at the level of the plane of the air gap, the section of this pole piece has a length and width on the order of several tenths of a micron. In this case, this same downstream pole piece is embodied with a shape that has a frontal constriction at the level of the air gap, for example in the manner described in U.S. Pat. No. 4,016,601, assigned to the Compagnie Internationale pour l'Informatique and entitled "Integrated Magnetic Head Having Pole-Pieces of a Reduced Frontal Width", issued on Apr. 5, 1977. The pole piece may equally well have a trapezoidal shape. In other words, this means that at the level of the air gap, the section of the downstream pole piece is much slighter in width than in the parts of this pole piece that are more remote from the recording carrier. In order to reduce the dimensions of the integrated transducer, it may be provided that no more than a limited number of conductors, indeed only a single conductor, be used, above all in the case of the writing of data.
The present trend in developing magneto-optical memories is toward attempting to write simultaneously on a plurality of tracks (8 tracks, for example), the eight-bit bytes of information being distributed for instance over eight adjacent tracks, in order to increase the rate at which the writing is performed and thereby reducing the time required for writing on one face of a disc. As a result, there is a need to realize an assembly of magnetic transducers of very small dimension, known as large scale integrated transducers, this assembly being known by the term "multi-transducer heads".
Multi-transducer heads are known, being described for instance in U.S. Pat. No. 4,198,667, issued on Apr. 15, 1980 and assigned to the Compagnie International pour l'Informatique CII Honeywell Bull, entitled "Magnetic Head Platform Incorporating at Least One Integrated Transducer". This type of head includes an assembly of transducers identical to those described in U.S. Pat. Nos. 4,016,601 and 4,287,544 mentioned above, disposed side by side in such a manner as to enable writing (or reading) data on a plurality of tracks simultaneously.
Other multi-transducer heads are also known. One such is described, for instance, in the article in "IEEE Transactions of Magnetics", Vol. MAG 18, No. 6, November 1982, on page 1140 by Wakapayashi, Abe and Miyairi. Each of the transducers of the multi-transducer heads mentioned herein includes a first conductor, called the "pre-polarization" conductor, which is common to all the transducers, and a second conductor in the form of a loop which is proper to each transducer and parallel to the first conductor. This second conductor is a selection conductor enabling the control of writing by the selected transducer for writing a given datum at a given instant on one track of the recording carrier.
The first, pre-polarization conductor imparts to each of the pole pieces of each of the transducers a magnetic state called the pre-polarization state, such that the magnetic field created in the immediate vicinity of the air gap is insufficient for writing a datum on the portion of the carrier located facing this air gap at this instant, yet it is sufficient for limiting the energy to be furnished to the selection conductor during the writing process.
For writing a datum on the carrier by means of a given conductor, a current pulse is provided in the second conductor of this transducer which tends to create in the pole pieces of this transducer a supplementary polarization, called "super-polarization", which being added to the prepolarization brought about by the passage of the current within the first conductor enables the transducer to create a magnetic field in the vicinity of its air gap, the intensity of which suffices for writing a datum on the part of the carrier located facing this air gap. The advantage of such a provision is a reduction in the intensity of the writing current in the winding, and particularly in the first conductor, which makes it possible to reduce the heating up of the heads and to reduce the number of turns of each winding, which is an advantage with respect to the multi-transducer heads mentioned above and described in U.S. Pat. No. 4,198,667, also mentioned above. Conversely, however, these heads have the disadvantage of having transducers the dimensions of which are such (essentially because of the loops embodying the second conductors) that the spacing between the different transducers (that is, the dimensions between the axes of symmetry of the air gaps normal to the carrier, and parallel to the direction of rotation of the carrier) is greater than the spacing between the tracks on which writing is to be performed (this spacing being roughly equal to the distance between the axes of symmetry of the tracks), so that it becomes extremely complicated and time-consuming to write all the data on a disc.
Furthermore, the shape and dimension of the windings are such that the risks of interaction between the magnetic fields created by the windings of adjacent transducers are not merely negligible; there is the threat of crosstalk occurring between adjacent transducers.